Prior art location determination systems, such as Global Positioning System (GPS), GLONASS, Loran, and the like, are known. Such systems rely, for the most part, on triangulation. In other words, these prior art location determination systems use two or more transmitters or receivers ("locators") located at distant positions. The locators are capable of transmitting or receiving signals to or from a locatable unit residing at a location to be determined. When given the locators' positions, the position of the locatable unit may be determined essentially by triangulation after measuring characteristics of the signals communicated between the locatable unit and the two locators.
Such prior art location determination systems achieve adequate performance for many different applications. However, they fail to provide an adequate solution for location determination in connection other applications, such as a substantially global radio telecommunications system that can provide communication services to substantially any point on or near the surface of the earth. Such a radio telecommunications system desirably operates with portable subscriber units. In order for subscriber units to have acceptable portability, they are desirably capable of low power battery operation, and they are desirably capable of transmitting and receiving electromagnetic signals through a relatively small antenna.
While subscriber units could be configured to incorporate components which take advantage of existing location determination systems, these components would substantially increase costs of the subscriber units. Moreover, relying on existing location determination systems could reduce reliability of a radio telecommunications system by introducing reliance upon an external system.
The techniques used by prior art location determination systems might possibly be incorporated into a radio telecommunications system, but the introduction of such techniques could seriously degrade communication services. For example, the requirement for two or more locators to be within view over the entire globe when combined with a global telecommunication capability, makes this approach undully cumbersome. While this requirement might be met by placing satellites in high or geosynchronous orbits around the earth, higher orbits place satellites further away from subscriber equipment on the earth. This larger distance causes the subscriber equipment to consume excessive power or incorporate larger antennas just to participate in communication services. Moreover, higher orbits require increased spectrum allocation to carry a given amount of communications because the allocated spectrum may be reused less frequently in a given area.